Vanishing-Edge and Perimeter-Overflow Surge Tank Balancing

Why this matters

A vanishing-edge (infinity-edge) or perimeter-overflow pool runs as two coupled vessels: the upper pool, which is intentionally flooded over a weir, and the lower catch basin or surge tank, which buffers the cascade. Get the flow balance wrong and the upper pool either runs dry at the weir (the edge effect disappears the moment a swimmer enters) or the catch basin overtops and pumps suck air. The hydraulic design is mechanical, not aesthetic, and the service tech is usually the only person on site who understands it after the builder leaves. ANSI/APSP/ICC-9 Standard for Aquatic Recreation Facilities sets the surge-volume math; this article translates it into a field-balancing procedure.

How the system works

• The edge pump (sometimes called the catch-basin pump or transfer pump) draws from the surge tank and pushes water back into the upper pool, usually through floor returns near the edge.
• Water sheets across the weir wall at a rate set by the edge-pump flow and the weir-wall geometry. A clean sheet typically wants 8 to 12 gpm per linear foot of weir for a continuous visual effect.
• The catch basin or trough collects the overflow and feeds the surge tank.
• The surge tank must hold enough volume to absorb the bather-displacement surge plus the upper-pool water that is "in transit" across the weir at any moment, without overflowing or starving the edge pump.

Sizing checks before you touch a valve

1. Measure the weir length in linear feet.
2. Target edge-pump flow: weir length x 10 gpm/ft as a starting point. Adjust within 8 to 12 gpm/ft to dial in the visual sheet.
3. Calculate bather surge: ANSI/APSP/ICC-9 uses one cubic foot displacement per bather as a planning value (a person displaces roughly 7.5 gallons when fully immersed). Multiply by maximum bather load.
4. Add weir transit volume: weir length x weir thickness x sheet depth, converted to gallons.
5. Add a freeboard reserve of 10 percent.
6. If the existing surge tank is smaller than the sum, you have a builder defect, not a balance problem. Flag to the owner.

Field balancing procedure

1. Start with the pool at static (no bather) level, edge pump off, main filter pump off.
2. Fill the upper pool until water just begins to sheet over the weir. Mark this elevation on the catch basin or trough wall.
3. Mark the surge tank level. This is the static surge tank elevation.
4. Start the main filter pump (skimmer/main-drain circulation) only. Confirm circulation does not pull the upper pool below the weir lip. If it does, the filter return geometry is sending too much suction to the upper pool; rebalance returns or add an autofill compensator.
5. Start the edge pump. Watch the surge tank level drop as water transits to the upper pool and back. Adjust the edge pump throttle valve (or VFD setpoint) until the sheet is clean across the full weir length without surge-tank starve.
6. Walk the weir; look for high spots (a dry stripe) and low spots (a torrent). High spots usually mean the weir wall has settled or was finished proud; only a mason fixes that.
7. With the edge pump running, simulate bather load by displacing a known volume into the upper pool (a 55-gallon drum of water from the auxiliary fill works). Watch the surge tank rise. It must not crest the overflow standpipe or drain to waste.

Edge-pump intake protection

• Surge tanks need a low-level cutoff that kills the edge pump before the suction breaks. A float switch or ultrasonic sensor wired through the pump contactor is the standard. Verify operation every service visit; a stuck float is the most common cause of a dry-run edge pump and a $4,000 mechanical seal failure.
• Strainer baskets on the edge pump must be inspected weekly on a commercial install; leaf-and-hair-and-lint baskets clog faster on infinity systems because the entire weir overflow concentrates debris into the catch basin.

Automation and chemical feed coupling

• The chemistry sensor manifold should be plumbed off the main filter loop, not the edge loop. The edge loop sees rapid level swings that produce nuisance ORP excursions.
• Salt-chlorine generator cells should be installed downstream of the filter, not in the edge return; the cell needs steady flow to avoid the low-flow lockout.

Commissioning handoff documentation

Leave the homeowner or facility a one-page sheet listing the weir length, target edge-pump gpm, throttle-valve position or VFD Hz setpoint, surge-tank static level, surge-tank high-level alarm setpoint, and edge-pump low-level cutoff setpoint. This is the only record that survives an ownership change.

References

• ANSI/APSP/ICC-9 American National Standard for Aquatic Recreation Facilities.
• Pentair IntelliFlo VSF Variable Speed and Flow Pump Installation Manual.
• Hayward TriStar VS Variable Speed Pump Owner's Manual.
• Pool & Hot Tub Alliance Certified Maintenance Specialist (CMS) curriculum, perimeter-overflow module.